Nutrient Cycles
What Are the Processes and Features of the Carbon Cycle?
The Carbon Cycle
Teacher Note: Misconception
Students may think that the carbon cycle consists only of photosynthesis and respiration, and that only animals carry out cellular respiration. The carbon cycle involves many processes in addition to photosynthesis and cellular respiration, such as combustion and decomposition. Organisms such as plants undergo both photosynthesis and cellular respiration.
Carbon is present in all living things and is the fundamental element of all organic compounds. All organisms depend on organic compounds such as fats and sugars for energy.
Organisms that make their own food from simple substances are called autotrophs. Most of these organisms, such as plants, algae, and some bacteriaglossary term (opens in a new window) and archaeaglossary term (opens in a new window), photosynthesize carbon-based food molecules using carbon dioxide from the atmosphere. Heterotrophs, on the other hand, depend on autotrophs, either directly or indirectly, for their food supply—they eat other organisms. The process of cellular respiration breaks down carbon-based compounds to yield energy. Carbon dioxide gas is given off in this process and is released from the organism back into the atmosphere.
The carbon cycleglossary term (opens in a new window) refers to the cycling of carbon through the environment. This includes cycling from reservoirs (rocks, sediments, and the ocean), to the atmosphere (mostly as CO2), to organisms in food webs, and back to reservoirs.
Carbon moves through the carbon cycle by multiple processes. Photosynthesis removes carbon dioxide from the atmosphere and uses it to build food molecules. Cellular respiration involves the breakdown of these food molecules, resulting in the release of carbon dioxide gas.
Organisms store carbon compounds in their bodies. When the organisms die, they decompose. Through the process of decomposition, much of their carbon is returned to the atmosphere as carbon dioxide. Some of this carbon is locked up in sedimentary rocks, such as limestone or chalk. Thus, the weathering of limestone rock, or its use in cement manufacture, contributes to carbon transfer through the carbon cycle. Some of this buried carbon forms fossil fuels such as coal, gas, and oil. The burning of fossil fuels releases carbon dioxide into the atmosphere.
Dead organisms occasionally become trapped under certain conditions that do not allow decomposition to take place. In these situations, the carbon from these organisms becomes buried deep within Earth.
Volcanic eruptions also release carbon dioxide into the atmosphere.
Many human activities, such as burning fossil fuels and forests, produce carbon dioxide. For a few thousand years before the Industrial Revolutionglossary term (opens in a new window), the amount of carbon dioxide in the atmosphere remained fairly constant. However, it is estimated that atmospheric carbon dioxide has increased by about 35% since the Industrial Revolutionglossary term (opens in a new window) around the early 19th century. This increase closely matches humankind’s increasing dependence on burning fossil fuels. The increase in atmospheric carbon dioxide is closely matched by a recent rise in average global temperatures. Why does this relationship exist?
Carbon dioxide is called a greenhouse gas because it has the property of retaining radiant heat. The presence of increased concentrations of greenhouse gases like carbon dioxide in Earth’s atmosphere therefore causes average global air temperatures to rise. Nowadays about half the carbon dioxide produced by natural and human processes is remaining in the atmosphere. So human activities, even when other factors are taken into account, are contributing to the trend of increasing global temperature.
Teacher Note: Connections
This item requires students to create a model of the carbon cycle by tracking the movement of a carbon atom through the cycle. They use models to simulate the flow of matter within and between systems at different scales. Help students see how carbon moves through the environment by using the Paper Slide strategy. Have them create a story about the movement of carbon in the environment. The Paper Slide strategy is found on the Professional Learning tab. Click on Strategies & Resources, then Spotlight On Strategies (SOS). Paper Slide is found underneath Research.
